Northward migration of the Javanese volcanic arc along thrust faults

Abstract

East Java is characterised by a complex interaction of volcanic and tectonic processes and it is marked by isolated eruptive centres scattered across the back-arc sedimentary basins. In 2006 a large sediment hosted geothermal system named Lusi, pierced the Kendeng basin in East Java and since then it continues in a relentless eruption of mud breccia. To investigate the spatial and structural relationships between the volcanic arc and the back-arc domains, we perform a local earthquake tomography. The inversion of regional earthquakes recorded by our seismic network (for about two years) shows sharp Vp and Vp/Vs transitions. We observe a marked reduction of P-wave velocities and a high Vp/Vs ratio in the back-arc basins. Our study highlights a clear connection between the plumbing system of the volcanic arc and the northern sedimentary province. We propose a conceptual model suggesting that magmas and hydrothermal fluids may migrate from the middle to the upper crust into the sedimentary basins capitalising on existing thrust faults. Such low angle faults, promoted by the compressional regime of the region, link the magmatic domain to the northern sedimentary provinces. This process may represent the early phase of volcanic arc migration when magma-derived fluids are focused into fractured and permeable geological structures. Our conceptual model would not only help to understand the occurrence of the abundant mantle-derived fluids sampled across the back-arc, but it is also consistent with the occurrence of isolated magmatic and hybrid systems piercing across sedimentary environments in the back-arc of Java.